The response to spectral light of rhesus monkeys, as a function of different adaptive states, will be measured by two techniques: the increment-threshold spectral sensitivity, a behavioral measure, and single unit extra-cellular recording from the visual pathways. The objective of this program is to apply these techniques simultaneously in the unanesthetized, performing subject in order to answer a series of systematic questions on the spectral information processing of the nervous system. Some of these questions concern: the role of opponent and non-opponent cells in spectral sensitivity, the absolute quantum threshold of single units and of behavioral thresholds, and the role of opponent cells in determining the peaks found at high intensity in behavioral data.